Post by plutronus on Oct 13, 2020 20:01:38 GMT
The development and implementation of air conditioning, Part 1: Heat principles
October 12, 2020 By Bill Schweber
From: EE World Online
We take air conditioning (cooling plus humidity control) for granted, and its development is largely due to one man’s “flash” of insight related to thermal principles and basic physics.
It’s hard for us today, with the ubiquitous air conditioning (A/C) and mechanical refrigeration we enjoy effortlessly, to grasp how A/C changed our lives: it made places like Florida habitable, and it gave us the unimaginable luxury of having access to cooling on demand for food, medication, electronic systems, and science research. It is based on mechanical refrigeration technology, of course, but is much more than that. In Paul Theroux’s novel, The Mosquito Coast, the protagonist (a role played by Harrison Ford in the fairly faithful 1986 movie version of the book) was determined to build an ammonia-based gas-absorption refrigeration plant in the tropical jungle to bring ice to natives who had never seen it and couldn’t imagine what it meant; he loudly proclaimed, “Refrigeration is civilization!” and there is considerable truth to that statement.
The development of air conditioning based on refrigeration principles and technology is largely due to the efforts of Willis Carrier, a Cornell University mechanical-engineering graduate who realized how to adapt compressors and their thermal cycle to cooling air and its moisture (humidity). Although we primarily associate A/C with personal comfort, he was a heating-system engineer for Buffalo Forge Company, working at Sackett & Wilhelms Lithography and Printing Company in Brooklyn, New York on the problem of inconsistent color registration due to temperature and humidity variations which affected paper dimensions, stiffness, and ink absorption.
Air conditioning systems also demonstrate rough equivalent to Moore’s well-known “law” for ICs: while the first A/C system weighed over two tons, occupied an entire section of a department-store floor, and cost the equivalent of tens of thousands of dollars, you can now buy a basic unit (admittedly far less powerful) for around $100, weighing about 50 pounds, and which fits in a window (Figure 1). Of course, there are also smaller units which fit in even compact cars – an amazing yet “taken for granted” advance.
It’s hard for us today, with the ubiquitous air conditioning (A/C) and mechanical refrigeration we enjoy effortlessly, to grasp how A/C changed our lives: it made places like Florida habitable, and it gave us the unimaginable luxury of having access to cooling on demand for food, medication, electronic systems, and science research. It is based on mechanical refrigeration technology, of course, but is much more than that. In Paul Theroux’s novel, The Mosquito Coast, the protagonist (a role played by Harrison Ford in the fairly faithful 1986 movie version of the book) was determined to build an ammonia-based gas-absorption refrigeration plant in the tropical jungle to bring ice to natives who had never seen it and couldn’t imagine what it meant; he loudly proclaimed, “Refrigeration is civilization!” and there is considerable truth to that statement.
The development of air conditioning based on refrigeration principles and technology is largely due to the efforts of Willis Carrier, a Cornell University mechanical-engineering graduate who realized how to adapt compressors and their thermal cycle to cooling air and its moisture (humidity). Although we primarily associate A/C with personal comfort, he was a heating-system engineer for Buffalo Forge Company, working at Sackett & Wilhelms Lithography and Printing Company in Brooklyn, New York on the problem of inconsistent color registration due to temperature and humidity variations which affected paper dimensions, stiffness, and ink absorption.
Air conditioning systems also demonstrate rough equivalent to Moore’s well-known “law” for ICs: while the first A/C system weighed over two tons, occupied an entire section of a department-store floor, and cost the equivalent of tens of thousands of dollars, you can now buy a basic unit (admittedly far less powerful) for around $100, weighing about 50 pounds, and which fits in a window (Figure 1). Of course, there are also smaller units which fit in even compact cars – an amazing yet “taken for granted” advance.
A brief review of heat
Before we look at how air conditioning works, it is useful to review heat concepts. While most electrical engineers are familiar with heat-related principles such as thermal resistance, cooling airflow, forced and unforced air cooling, conduction, convection, radiation, and heat transfer, those aspects of thermal physics are not the only ones needed here.
The second law of thermodynamics is clear: heat (which is a manifestation of “work” in physics) flows only from warmer bodies to cooler ones, in a process called heat transfer. While all substances can absorb heat, refrigerants can absorb heat quickly and in large amounts. Common refrigerants to lesser or greater degrees are air, water, brine, ice, ammonia, carbon dioxide, and fluorocarbons or chlorofluorocarbons (Freon is the best known of these, although its use is now largely restricted due to concerns about its effects when released in the atmosphere).
The energy content of air is called enthalpy, and it is designated in British thermal units (BTUs) per pound or kilojoules per kilogram of dry air in the SI system. A gas gives up its heat when it changes phase to the liquid state (and a liquid gives up its heat when it transitions to a solid); in the reverse process of vaporization, the liquid loses heat as it changes phase to a gas. The specific temperature at which the liquid changes to gas at a specified pressure is defined as its boiling point. The heat that a material gains or loses during condensation, vaporization, freezing, or melting is called latent heat.
In addition to the liquid-gas-liquid phase change used in the A/C system, there is the well-known liquid-to-solid (and reverse) phase change. Removing enough heat from a liquid will cause it to freeze, of course, while adding heat to a solid causes it to melt. This is why citrus growers spray water on their crops to protect them from frost as the temperature drops below freezing, although this action seems counterintuitive to most people: as the water freezes, it gives up its latent heat and warms the fruit, hopefully saving it from harm.
Before we look at how air conditioning works, it is useful to review heat concepts. While most electrical engineers are familiar with heat-related principles such as thermal resistance, cooling airflow, forced and unforced air cooling, conduction, convection, radiation, and heat transfer, those aspects of thermal physics are not the only ones needed here.
The second law of thermodynamics is clear: heat (which is a manifestation of “work” in physics) flows only from warmer bodies to cooler ones, in a process called heat transfer. While all substances can absorb heat, refrigerants can absorb heat quickly and in large amounts. Common refrigerants to lesser or greater degrees are air, water, brine, ice, ammonia, carbon dioxide, and fluorocarbons or chlorofluorocarbons (Freon is the best known of these, although its use is now largely restricted due to concerns about its effects when released in the atmosphere).
The energy content of air is called enthalpy, and it is designated in British thermal units (BTUs) per pound or kilojoules per kilogram of dry air in the SI system. A gas gives up its heat when it changes phase to the liquid state (and a liquid gives up its heat when it transitions to a solid); in the reverse process of vaporization, the liquid loses heat as it changes phase to a gas. The specific temperature at which the liquid changes to gas at a specified pressure is defined as its boiling point. The heat that a material gains or loses during condensation, vaporization, freezing, or melting is called latent heat.
In addition to the liquid-gas-liquid phase change used in the A/C system, there is the well-known liquid-to-solid (and reverse) phase change. Removing enough heat from a liquid will cause it to freeze, of course, while adding heat to a solid causes it to melt. This is why citrus growers spray water on their crops to protect them from frost as the temperature drops below freezing, although this action seems counterintuitive to most people: as the water freezes, it gives up its latent heat and warms the fruit, hopefully saving it from harm.
To read the remainder of this two part series see:
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